best available copv



Patented June 12, 1923.

UNITED STATES PATENT OFFICE.

rLomcEL A. noms, oF New Yemen. Y.

RHEOSTAT.

Application lcd November 18, 1921. Serial No. 516,134.

To all whom it may concern:

Be it known that I, Fnomoml A. ROJA'S, a citizen of the Republic of Santo Domingo, residing at New York city, in the county of Bronx, State of New York, have invented cert-ain new and useful Improvements in Rheostats; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the saine.

This invention relates to rheostats of the comprcssiblc resistance type, and has for its objects, first. to eti'ect certain general improvements in such rheostats, and second, to adapt them for use within standard electric light sockets, in order that they may be used for dimming electric lights, regulating the current in small electrical instruments, and in general serve to control the current in all devices commonly attached to such sockets.

The resistance element of a rheostat of this type consists in general of a fibrous, non-conducting material, such as asbestos, impregnated with a powdered conducting substance, such as graphite, and held between two relatively movable members. As these members are moved toward each other the material is compressed and its resistance decreased. "As the 4members are moved apart the material is expanded and its resistance increased.

In accordance with my Patents Nos. 1,315,579, september 9, 1919, and 1,366,945, February 1, 1921, the relatively movable members are in the form of conducting plates or electrodes to which the lresistance material is secured, and is thus positively expanded as well as compressed by movement of the plates, no reliance being placed upon thc resiliency of the material itself t0 effect expansion.` It hasbeen found that. due to the internal frictional resistance of the material itself, and the frictional resistance of the container walls, the greater part of the expansion and compression tends to take place in thc material adjacent the movable plate (in practice one late is commonly fixed and the other movab e). This results in an overheating of'this part of the resistance material with consequent danger .if burningr ont. It also affects the nicety of regulation. -In accordance with the present invention this objection has heeirorerconic by increasing the proportion of conductingr material in that part of the resistance ele; ment adjacent the movable plate. 4

I have also noted that after long use, the infinitude of minute arcs gradually burn up the graphitepowder and finally render the resistance material useless. In accordance with the present invention I have discovered that. the addition of a small amount of powdered copper to the graphite cuts down this consumption, and materially increases the life of the resistance.

In rheostats of the compress'ible resistance type it is common to employ contact pins which extend inwardly from one or both .0f the plates and, by contacting Witheaeh other or with the other plate, form a shunt invparallcl with the resistance which cuts out the latter, and allows full current to How when the compressionof the resistancel material has reached a predetermined point.. VAs long, however, as these contact pins pass through 4the resistance material itself, the active part of the resistance is that which is included between the inner ends of the pins',

and that part adjacent the plates is practivention the shunt is completely insulated :c

from the 'resistance material, so`that, the whole of the latter is effective :until the shunt is thrown in. I have alsoI devised `meanswhich laudibly' indicate, whenpthe shunt has been or is about to be,'close d, so that the loperator knows when thejjesistance hasbeen thrown out and ulleu'rrentis Howinfv.

In my Patent No. 1,366,945 l have shown external fixed resistances designed .to be thrown in parallel with the rheo'stat.y In acf cordance with thebpreresentr invention Ihave provided means for adding fixed resistancs in series. thus making it possible to regulate the total potential drop between the .terminals of the compressible material. This ybecomes very desirable when the rheostat has been adapted for use within a standard electric light socket. These fixed lresistances may be added in the4 shape of discs of refractory non-conducting material carrying.iin-. bedded particles of conducting matter, the discs being placed between the compressible resistance and one ofthe plates.

To design a rlieostat of he compressible resistance type, so that it can be built within a standard lamp socket, has been diiiicult. The ver bulkiness of the resistance material which makes it safe also makes it difiicult to work with when space is eX- tremely limited` The small space permitted the resistance material within the socket is further limited by the passage of the return lead through it, and by the provision of the shunt which shall carry the full current when the conductivity of the resistance approaches full current capacity. These considerations have made necessary the provision of a thin-walled container occupying thel least practicable space and shaped to accommodate both the return lead and the shunt. This container must also, of course, be nonconducting and heat-resisting. Other structural changes have been made which will be better described in connection with the accompanying drawings, which illustrate a lamp socket equipped `with a rheostat in accordance with the invention. Although the drawings show a preferred embodiment other forms are, of course1 possible and are to he considered as embraced within the invention as defined by the appended claims.

In these drawings Fig. 1 is a vertical section through a socket and its rheostat; Fig. 2 is a vertical section taken along line 2-2 of Fig. 1 and at 90 to the section shown in Fig. 1; Figs. B, 4, 5 and G are transverse sections taken along lines 3 3, 4-4, 5-5 and 6-6 respectively of Fig. 1; Figs. 7, 8, 9 and 10 are detailed views of paits which will b'edescribed later; andrFig. 11 is a fragmentary View showing a fixed resistance disc in position.

The socket itself, 'which is of standard size and' form, is indicated'in the drawings by reference numeral 1. The socket is r0- vided at its lower end with the usual ase cup 2, adapted to be screwed upon al supporting terminal 3, from which the two leads 4 yand 5 extend, and at its outer end with a threaded seat 6 for an attachment. A lamp '7 has been shown and the operation of the rheostat will be described in connection with a lamp, but its use is1 of course, not so limited. This seat 6 forms one of the electrical contacts with the lamp and is insulted'from the socket by means of a cylindrical wall of insulating material 8. The foregoing is common construction and is found in practically every electric light socket.

In accordance with this invention a rheostat is mounted Within the socket and placed in series with the lamp. The resistance ele ment of this rheostat comprises a compressible mixture of asbestos fibres and conducting powders which shall be discussed in de tail later. It is indicated by the reference numeral 9 and is held between two relatively movable metal plates ll and l2 and within a. container 13, preferably of cnameled iron. A container of enameled metal has been found best adapted, for not only is it heat-resistant and non-conducting, but its walls can be made very thin and hence occupy a minimum of valuable space. lt also has a certain flexibility and can adapt itselfto slight variations of the other partsl from standard size. By coating the inner walls of this container with a vitreous enamel another advantage is gained. lVhen graphite is used as the constituent of the conductin(7 element of the resistance, it has been note( that it tends to form a coating over the surface of the container walls, which coating being electrically conductive acts to short circuit the current around the resistance and causes trouble. Now l have discovered that if Ithe container walls are made of glass or coatedwith vitreous enamel this diiiiculty is overcome and the graphite docs not form a coating. The container 13 and the lower plate 12 are supported upon a base 14. held within the base of the socket andinade of some insulating and heat-re sisting material1 preferably porcelain. The container 13, although of general cylindrical form, is irovided at one side with a recess 15 exten ing radially inward and beyond the vertical axis ofthe container, as is clearly shown in Fig. 6. The walls defining this Yrecess do not extend the entire height of the container., but terminate slight-ly above a half way point, as 'shown in Figs. 1, 2 and 10. 4 The function of this recess is to house the other lead and the shunt, which is thrown in parallel with the resistance whenthe conductvity'of the latter approaches full current capacity, and to insi 1late them cornpletely from the resistance material. Secured to the lower face of the upper plate 11 is a block 16 ofI non-conducting heat-resisting material, preferably porcelain, which extends downwardly and is designed to fit within theirecess 15 and exclude the resistr ance material. This block is provided wit-h a boss 147 which extends through a central aperture 4in the upper plate 11. The block 16 is secured to the plate 11 by means of a metal screw 18, which makes electrical contact with the plate 11,`the block being countersunk at 19 to provide a. cavity, the fune tion of which. will be described later. The block 16 is also provided with a passageway 21, which coincides with the vertical axis of the container 13. The position of block 16 within recess 15 prevents rotation of plate 11. as will be evident.

The seat 6 for the lamp is provided with an externally threaded, depending extension 22 of smaller diameter. Within this exten` sion is mounted a block 23, preferably of porcelain, which forms the hase of the lamp seat. The lower end ol' this block 23 pro jects slightly' beyond the extension 22 for n purpose which will be apparent later. Contact with the lower lamp terminal must be made at the center of this base and the block 23 is therefore provided with a central aperture through which a metallic pin 24 is passed. This pin extends loosely through the passage 21 in the block 16 and through a central aperture 25 in the base 14. Here it terminates in a metal plate 10 and makes Contact with the lead .4, .The base 14 is provided at one side with a pin 26, which extends upwardly through the lower plate 12, making electrical contact therewith and into the recess 15. The lower end of this pin 26 bears aga-inst a plate 2O and makes Contact with lead 5. Pin 26 is situated directly below pin 18, as is clearly shown in Fig 1, but is normally spaced therefrom. Pin 26 thus serves to limit the downward movement of the plate 11.

An internally threaded collar 27 is mounted upon the extension 22. This collar 27 is composed of inner and outer concentric members, 28 and 29 respectively, spaced apart by a sheet of insulating material 31. The inner member 28 must be made of metal and makes electrical contact with the extension 22. The outer member 29 is generally made of metal and for this reason must be insulated from the innei` member, but it is, of course, clear that the outer member might be made of any suitable material. Surrounding the collar 27 is a chain 32. This chain is secured at its middle to the collar at 33 and extends a full turn in both directions about the collar, the two ends finally emerging together through an opening 34 in the socket. ,This collar 27 bears against the upper face of'the plate 11, the inner member 28 ,making electrical contact therewith, and downward movement of plate yIland compression of thev resistance material is effected through movement of rthis collar. Upward movement ofthe plateand expansion ofthe adhering resistance materia-l is e'ected b means of an expansion coil spring 35 encirc ing -pin 24 and bearing upon both base 14 and block 16.

The operation of. the device acting as an electric light dimmer is as follows: When the lamp 7 is in position, as; shown, and the leads 4 and'5 connected to a 'sourceof electrical energy, the path of the current is throughlead 4 plate 10, pin 24, the lamp, seat, extension 22, inner member 2.86ct the collar 27, plate 11, resistance material 9, plate 12` pin 26, plate 20 and lead'5. As is well understood, the conductivity of the resistance material 9 varies directly with its density. If, therefore` the chain 32 is pulled so as to cause the collar 27 to move downwardly, the plate 11 is moved toward plate 12, the material 9 is compressed and its con ductivity increases. This means that more and more current is allowed to pass through the lamp filament and the light grows brighter and brighter. Continuing the movement of' collar 27 in a downward direction a point is reached when the head of screw 18 makes contact with the uppercnd of pin 26, which has entered countersink 19. This immediately shunts the current through theseelements and makes it unnecessary for the resistance-to carry the whole load. By adjusting the space between the head of screw' 18 and the upper end-ofpin 26 relative to the resistance of material 9 it will happen that the resistance of this material will permit the How of sutlicientcurrent for practically full luminosity at the time the shunt circuit is thrown into parallel. This means that when contact is made the light gives a small flickerwhich indicates that full current is flowing. This flicker is but slight and unobjection'able and is in no sensef a jump, as would be the'case were'the resistance of material 9 considerably greater when contact is made. lVhen the rheostat is used to regulate the current flowing through the filament of an electric lamp, 'the flicker is probably sufficient indication that full cnrrent is owing. When, however', the attachment is an 'iron or a heatinl pad, means should be provided which sha1 give audible indication. ne such means comprises a short strip of metal 36 secured to the lower face of block 16 and extending Within'the line of pin 26. This strip of meta-l is designed to snap within a small cavity 37 at the'side of pin 26 at the time that contact with screw 18 is made, this snap being distinctly audible. If the other end ofchain 32 is pulled so that the collar 27 moves upwardly. the spring 35 will forcezplatel 11 away fromV plate 12, and as the resistance' material is firmly secured to both plates it is positively expanded'and its conductivity drops. lAs alresult the current passing through the filament decreases and the light isd'immedu -Bycontinuing the upwardjmovement ofthe collar 27 a point Will-'be reached when -upward movement of the plate? 11 will be arrested by block 23. Further turning of -'collar' 27 breaks its contact with-'plate 11 and the light is' extinguished; This break shouldcome when the conductivity of the resistance material has'been reduced s o low that the lamp filament is barely glowing.` `TheI light is therefore very gently' extinguished. Y By providing stops which shall limitV the upward movement of the `collar shortlyfafter contact is broken, t.he"operator will'be apprised of this fact when working wit-hany attachment which-does not glow and hence give visible indication that the current is approaching zero. V

I have found that4 amixturc of equal parts of asbestos fibre,`andpowdered graphite, forms `an excellent `compressihl'e resistance material. Tt has been' found` however, as stated above, that after several years usage the infinitude of minute arcs tend to consume the graphite particles' and that the resistance material deteriora'itesfA I haveV found thatl if theigraphitef is' mixed with copper powder' in=theproportions of about 98 parts ofgraph-ite to 2 partsof copper, this consumption of thelgraphitefis les# sened and the life of the materiali'is 4greatly prolonged. The addition=-of asma'll quantity of calcium' carbonate; is also helpful. l

As stated above; -Wher'efonly'one of'the contact platesis imoifable, the -elect of: the pull 'upon the-'resistance 'material1= im'medi# ately adjacent tliat"plateis greaterlthan uponfthat adj acntf-thestationary plate; the resistance of the container'wal-lsan'dthedn ternalresistance'fof the material-itself' reventinguniform"transmissionof'the pu l to all parts of the material;HTliisfineans that withl a uniform mixture the lpotenti'al'drop is greater iii-th upp'er half than in the lower 'Whenthematerial isexpanded.: Th-is,.ot

course,` prevents Ya--regngfulationias 'delicate as that whichlmight be obtafined with-fa uniform potential :and results 1in overheatingfo the upper part of the resistance material. It has been found that this difficulty can be obviated by inakingthat' half of thematerial contacting with' `the .movable plate richer in conducting material than that half contacting with the stationary plate. For example, fWhere powdered graphite is used as the conducting' material andfa'sbestosffibre asthenon-"conducting and yieldable material, that half `ot the resistance element contactng with .the. movable: .plate is composed oasbestosvfibre; ffour. parts, and graphite powder, seven parts, 'whilethe-other hal-f is composed of4 equal parts of fibre and graphite."

`ln Fig. 11a fixed vresistance element in the form of a disc 38 has been shown. This disc may be made of'any'eompara-tively refractory,I inonconducting paste or cement in which conduct-ing material is imbedded. By placing it upon the lower plate v12 and sec'urin it tothe resistance material any number 0% volts may be dropped before the current enters the resistance and thus a heat loss which mightbe injuriousto the compressible materia-l can be dissipated in-:the fixed resistance. Forexainple, by the use of such a disc it would be possible toiburn a 110 volt lamp on a220 volt circuit.

It is desirable. that such a dise .barcav zero temperature Coellicient. ,Carbon having a negative coeilicient, its conductivity increasing with increase in temperature, and metals having` a positive coelliciont, a

mixture of carbon and metals canwbe made, the conductivity of which shall be substantially unaffected by temperature changes. However. the rate at which the conductivity of metals changes with temper ature is much lcss than the rate of 4change for carbon, and .for this reason in a mixture of carbon and metals having al'zero coelicient, the metal constituent must largely 'predominate. This is objectionable because a predominance ofmetals tends to short-circuit therheostat, 4and because no easily oxidizable metals can be used, and the cost of those relatively non-oxidif/,able metals, such as copper, silveiggold, platinum, etc., is prohibitive. 'If'a non-oxidizable, inexpensive material could be found having'a. high negative coefficient, and which would not tend to shortc-ircuit the rheostat, such a material would be much superior to Vmetals in making a `mixture of zero coefliciency. Such a material iserystalline, or conductive silicon. l therefore make' this disc of a 'mixture of carbon in any :of its allotropieformswith crystallline," or conductive silicon, imbedding:r the mixture in any relatively refractory-noneonducting cement. A satisfactory composition' for this purpose is, stated iny approximate proportions:

" Silicon' parts by weight.

Graphite 2 parts bv Weight..

Cement 4 part-s by Weight It is, of course, obvious that the compressible resistance can similarly be made of a suhstantially zero temperature 'coefficient by the addition ofconductivesi'licon Thus, a inaterial analogous tothe above hut substituting asbestos for cement' would Vhave a suhstaiitia'lly zerotemperature coefficient. A satisfactory adhesive for use in attaching the resistance material to the plates is a mixture of a heavy oil and a powdered graphite, although, of course, other conducting adhesives or mechanicalrneans might he employed. The adhesion of the resistance material to the plates is essential to the operation of the rheostat, as without this. the resistance', although it can be decreased by compression, cannot `well be increased. lt has been 'found that the resistance material adheres best to iron, the slight oxide ihn probably servingr successfully to anchor the adhesive.

There is, of course, the .usual heat'loss when current passes through the compressib'le resistance material, the loss being at its maximum when the conductivity of the resistance equals that of the external resistance, such as a lamp filament. For high and low resistances this loss is very little. Even at its -inaxiniunn however, the heat generated is successfully radiated, so that the heating of the socket is not excessive` Nevertheless, I have found it 'nest to provide the leads -t and 5 with an asbestos coveringr for the first feniuches of their length. This heating ol the resistance vmaterial expands it increases the contact amongv the conductingY particles, and thus decreases the resistance of the .vliole. ln my Patent 1.366,945. l have disclosed certain thermostatic means for comn lll] pensating for this change in resistance. In the present structure the metal rod 24 and the enainelled ii'on container 13 act as thermost-ats. lfVith rise in temperatuie the rml expands and thus increases the distance between plates 11 and 12 while the expansion of the container tends t close in upon the recess 15 and increase the volume 'of the resistance material. These changes can be made to compensate for the expansion of the resistance material and the resulting decrease in its resistance.

Voi)

This rheostat is free from many of thc defects of others of the compi'essible resistance type. Unequal expansion of the material is compensated for deterioration of the material through consiunption of gra hite is lessened; and a shunt is provided willich is completely insulated from the material. It has also been successfully adapted for use within a standard electric light socket and provided with simple means for eil'ect-ing compression and expansion of the material, for indicating when full current is liowiin,r and when the current has been cnt off. This rheostat is also peculiarly ada ted for use in connection'with dimming individual electric lights. The current can be accurately controll'ed and with such a nlcety that no steps whatever are noted as the light is turned up oi'id'own. The entire mechanism having been successfully mounted within a socket of standard size, is therefore a decided improvement overthose devices which require special sockets. Furthermore, this device requires n'ospecal 1am but 'operates on any of the 1 incandescent ectric lampsnow on the marliet. Each rheostat is, of course, designed for a lampof a given wattage, the' resistance lof the resistance material when half comressed being equal to the resistance of the amp filament. Lamps of higher wattage may, lioweverrbe used without immediate danger of burning out, and 'in this respect 1 the rheostat'of the present invention possesses an advantageover the wire 'coil rheostats emloyed for similar purposes, Lower powered liimps' may' also be employed without sacrificing the use of the 4lowest. lights. It is ob vious, however, that' its use is not confined to electric light dimming. It ma)r well be employed for many and various purposes. Although one preferred embodiment of the invention has been shown'and described it is clear that other forms might be devised which would operate perhaps equally well. The invention is therefore not limited by the precise structural details shown and described I claim: 1. A rheostat comprising the combination of a movable member and coinpressble resistance material secured to said member.Y the conductivit of the resistance material immediately adjacent the member being normail)r renter than that of the rest of the materia 2. A rheostat com rising the combination of a stutionar)r mem r, a'inovable member, and compressi .ile resistance material between .said members and secured thereto, the con ductivity of the resistance material ad'acent the movable member being normally renter than that of the material adjacent tie stationary member.

3. A rheostat com )rising the combination of a stationary mem r, a. movable member, and a compressible resistance material comprising a non-conducting fibrous material impregnated with conducting material between said members and secured thereto, the rat-io of conducting to non-conducting material being higher adjacent the movable member.

Ll. A. rheostat com )rising the combination of a stationary mem er, a movable member, and a compreible resistance material comprising asbestos fibre impregnated' with owdered graphite between said plates an secured thereto, the ratio of Graphite to asbestos in that part of the resistance material adjacent the movable member being about seven to four, and that in the part adjacent the stationary member being about one to one.

5. A rheostat comprising the combination of two iron plates and a com ressible resist since material between and a hering to said plates.

6. A compressible resistance material com- )rising a non-conductin fibrous material impregnated with powered carbon and powdered copper.

7. A compressible resistance materialcomprising a non-conducting ibr'ousiuateral impregnated with a mixture of powdered graphite and copper. v

8. A eompressible resistance materialcom- )rising a non-conducting fbrous'material impregnated with a mixture of' powdered graphite, copper and calcium carbonate.

9. A compressible resistance material comprisng asbestos fibre imp mated with pow.- dered carbon and powdere copper.

10. A compressble resistance' rheostat provided with means for audibly indicating when the resistance of the material has reached a predetermined point.

11. A rlieostat comprising the combination of a compressible resistance material, means for compressing said material lio-vary its resistance2 a shunt designed to be thrown in parallel with the resistance material when the resistance of the latter hasreached a predetermined point. and means audibly indicat-ing when the shunt has been thrown in.

1Q. A rheostat comprising the combination of n compressible resistance material, means for compressing said material to va its resistance, and a shunt designed to be thrown in parallel with the resistance material when the resistance of the lutter has reached .a predetermined oint, said shunt being completely insulate from the resist ance material.

l. A rheostat comprising the combination of avcompressible resistance material held betweenftio conducting membersu container forthe resistance material, means for movin-r one of the members to vary the density un( resistance ofthe resistance material, and a shunt' outside. oi the container which is thrown in parallel with the resistance material when' the resistance of the latter reachesza` redetermned point. y

14.`A r eostat. comprising the combination of a compressible resistance material held between two conducting members, a. container for the resistance material, means for-,moving one of the members to vary thc density resistance of the resistance mab terial; a4 vrecess in. the container from which theiresistancc material is excluded, u pin in s'ad recess extending .inwardly from one of the 'members andgmnking electrical contact with the otherY member when the rcsistance of ",.the resistance materiel has reached a .predetermined point. "i 15.A rheostatfor use in dimming electric lights comprising the combination of u cprnpressible xjlesstaxiceA material, meansv for Vai-yin the density ofthe materialvto vary its con uctivit and control the current passing thruvh t e filament of the lamp,and meansin haztin when theV light is burning nieuwe@ lightness. ,i ;..ii6.'.A..heostat for. use ndimmingelectric lights comprising the combination of a compressible resistance material1 n ienns -for varyangJtlre. density; of the material` to vary xts i:,iyityx and control the currentassmg through the filament of the lamp-nn a lon resistzirxce;.nfhichisV thrown in arallel wltbresistancematerwl when. t 1e 'condactiyity ogftheletter hcsgbcemincreased to pme... .teneur maximum. humane, the ,sligh increase current flow upon clos- -lzgio nthehgtww *ai 11,., A r epstat comprisinv a compressible materinL. two re atively movable niembelfsbetyveen which the resistance material is'iheld andadiseotlixed resistance interposed: bettveen oneof the .membersend theresistnncezmaterial.

x].l3.''l1n .c' Vnil?inntion oan electric light socket; ,ze rheostat within the socket coma, lcom ressible. resistance, materiel, and a ;thin -'wa ed,- non-conducting, heatresisting container for the resistancexnaterral.

`A',Llhe combination ofv an electric light socket, a rheostat within theA socket comprisingacompressible resistance material, and afvitreous-enameledmetal container, for the resistance material.

.the .shuuntlresnlting Ain a. slight flicker 20. A compressible resistor, comprising a container and a com )rcssible resistance within the container nm e up of o nonconducting material and carbon, 'the side walls of the container being faced with a. material to which the carbon will not adhere to form a conducting coating.

21. A compressible resistor comprising a container and a compressihle resistance within the-container made up of a non-conducting material and carbon, the side walls of the container being faced with vitreous material.

22. In an electric licht socket, the combination of two leads tfrom the socket to a source of electrical ener;= a rheostat of compressible resistance material within the socket and included' in circuit with one of thc leads, an extension of the other lend passing through the, resistance material. l

23. In an electric light socket, the combixmtion of two relatively movahlcplates within the socket, u compressible resistance material between the plates, n screw-threaded collar bearing, nfninst one of the plates, and means for rotating the collar to movethe plate and compress the resistancomatezzial.

24. In an electric light sockct,`the combination of two relatively movable .plates within Athc socket, a, compressible resistance material betweenntheplates, a screw-:threaded collar-,Ineens constantljr rging one of ,the plates toirard'thc collar-,lend means for rotatin g thecollar-to move the `plate and -cornpress the resistnncernfrtnl'ial.y l 25;'111 an elc triclight socket, the'cn'ibination :of two contact terminals -fon; anxit techme'nt plug,'an lextension on ,onelof the termina La movable collar monntedon said extension, 'trio rclatirely moyaiblefplute's within,*the',iocketii ,e compressible material' between the'pl'ates, one' plete Icon'- tectiiig Withrthe collar, aA ixedmcpnductor leading from them-her, terminal and through the' resistance' materiel vbut insulated'y therefrom, leads froni'thesocket electrically con nectedwith the secondplate and the, lixed comluctor:A lsspecti'vely', andmeans for mov- .ingthe'collarto compress the rsistance ina,- .tcrial ancl'vary its conductivity.

A 26. In an electric light socket, the combination of a.' screx'v-'threadedsat for an vntvtaclnnentA Apliig constituting onei terminal, a

centrally'llocatedtpin constituting the other terminal, a compressihle resistance material eleqtricelly, connected with 4the seat, Aleads from-the socket electrically connected with the resistanceI material and the pin respectively, and means for varying the densit of the resistance material to vary its con uctivity. i

A 27. In an electric light socket, the combinaton of twoterminuls fr makin'gelectric contact with :inv attachment plug, one of said terminals being directlyconnected with Athe mova le member in one direction to move its plate and compress the resistance material, means for causing the plate to follow the movable member when the latter is retracted to expand the resistance material, und means for breaking contact between the movable member and its plate when the conductivity of the resistance material reaches :i predetermined point.

2S. In an electric li ht socket, the combination of two termina s for making electric Contact; with an attachment plug, one of said terminals being directly connected with an outside lend, an extension on the other terminal, e, movable member on said extension electrically contacting therewith, two relatively movable plates, compressible resistance material between and adhering to said plates,ithe seid movable member electrically contacting with one plate, means for moving the movable member in one direction to move its plate and compress the resistance material and pins on the two plates extending towurd each other and designed to contact when the conductivit of the compressible resistance material as reached a predetermined oint.

29. The com ination of an electric light1 socket designed to hold a lamp or other attachment, and u. compressible resistance rheostnt within the socket'in series with; the lamp or other attachment, the other lead being a metallic pin passing through .the rheostat, said )in acting as a. thermostat compensating or the expansion of the resistance material with heat.

30. A rheostat comprising lthe `conzibinetion of` e compressible resistance material, mea-ns for compressing said material to vary. its resistancez a, shunt designed to be thrown in parallel with the resistance materiel when the resistance of the latter has reached a predetermined oint, end means audibly indica-ting when t 1e resistance of the materiel has reached a predetermined point.

In testimony whereof Iellix my signature.

FLORICEL A. ROJAS. 

